The present invention relates to an apparatus for measuring the thickness of moving films; more particularly, the invention relates to a rotatable sensor which makes rotatable contact with a moving sheet of thin material. The most notable application of the invention is in connection with measuring thickness of thin plastic film in sheet form, of the type typically manufactured in blown film machines.
Most prior art thickness gauges used in connection with blown film machines utilize contact devices; i.e., a sensor is brought into contact with a moving sheet of plastic film and film thickness measurements are made as the film passes the sensor head. Some of these devices are transversely movable across or around a blown film bubble so as to traverse the entire circumferential surface area of the film bubble. In other cases, the sensor head is mounted in a fixed position and the bubble itself rotates transversely at a slow turning rate at the same time as the film moves longitudinally past the head.
Some prior art film thickness measuring devices do not actually contact the film, but these devices generally suffer from the disadvantage of poor accuracy, and such devices are generally unable to make accurate measurements of thickness, particularly of very thin films.
Examples of prior art film thickness measuring devices are found in prior patents. U.S. Pat. No. 3,764,899, issued Oct. 9, 1973, discloses an electrode assembly which is mounted in a fixed position, and a thin plastic film strip is movably passed across a sensing head to obtain measurements of film thickness in conjunction with an electronic circuit for processing the capacitance signals produced thereby. U.S. Pat. No. 4,947,131, issued Aug. 7, 1990, discloses a capacitance bar sensor designed to be placed in contacting relationship with a moving film surface, to provide extremely accurate film thickness measurements independent of temperature variations. U.S. Pat. No. 3,300,716, issued Jan. 4, 1967, discloses a capacitance sensing apparatus for measuring transversely across a sheet of moving film, particularly disclosing electronic circuits for enhancing the signal processing. Various circuits for processing signals from a capacitance sensor of the type disclosed herein are well known from the foregoing and other prior art patents, and such circuits are not specifically dealt with in the present disclosure.
One of the problems with prior art contact sensors is that, because such sensors inherently must remain in contact with a moving film, they tend to mark or scratch the film surface. Such sensors may be designed to minimize this problem, but in particular film applications such as high-quality optical films, even fine hairline scratches on the film may be cause for concern. Because of the relative differences in velocity between the sensor and the film, the problem of film scratching is always present with sensors of this type.